1. Field of the Invention
The present invention relates to a cylindrical motor and an optical apparatus using the motor.
2. Related Background Art
FIG. 17 is a longitudinal sectional view showing a structural example of a conventional stepping motor, and FIG. 18 is a partial sectional view schematically showing a state of a magnetic flux that flows from a stator of the stepping motor shown in FIG. 17.
In the figures, two bobbins 101 having stator coils 105 concentrically wound therearound are juxtaposed in an axial direction, and are held between and fixed to separate stator yokes 106, respectively. An inner diameter surface of each of the stator yokes 106 is formed with stator teeth 106a and 106b, which are alternately arranged in a circumferential direction of the inner diameter surface of the bobbin 101. The stator yokes 106, each of which is integrally formed with the stator tooth 106a or 106b, are fixed to two cases 103, respectively. Thus, there are formed two stators 102 respectively corresponding to the two stator coils 105 for excitation. A flange 115 and a bearing 108 are fixed to one of the two cases 103, and another bearing 108 is fixed to the other case 103. A rotor 109 is composed of a rotor magnet 111 fixed to a rotor shaft 110, and the rotor magnet 111 forms a radially extending gap portion with respect to the stator yoke 106 of each of the stators 102. The rotor shaft 110 is rotatably supported by the two bearings 108.
In the conventional compact stepping motor described above, the cases 103, the bobbins 101, the stator coils 105, and the stator yokes 106 are concentrically disposed in the outer circumference of the rotor 109, which raises a problem in that the motor is increased in outer dimensions. Further, as shown in FIG. 18, a magnetic flux, which is generated due to energization of the stator coils 105, mainly passes through an end face 106a1 of the stator tooth 106a and an end face 106b1 of the stator tooth 106b. Thus, there has been a problem in that the magnetic flux does not act on the rotor magnet 111 effectively, making it impossible to attain higher output of the motor.
With the objective of solving the above-mentioned problems, there has been proposed a motor with the structure as disclosed in Japanese Patent Application Laid-Open No. H09-331666 (U.S. Pat. No. 5,831,356). FIG. 19 is a longitudinal sectional view schematically showing the motor. In this figure, reference numeral 311 denotes a magnet; 312 denotes a first coil; 313 denotes a second coil; 314 denotes a first stator; 314a and 314b denote first outer magnetic pole portions; 314c and 314d denote first inner magnetic pole portions; 315 denotes a second stator; 315a and 315b denote second outer magnetic pole portions; 315c and 315d denote second inner magnetic pole portions; and 316 denotes a connecting ring for holding the first stator 314 and the second stator 315. Reference numeral 317 denotes an output shaft to which the magnet 311 is fastened and which rotates integrally with the magnet 311. The output shaft 317 is rotatably supported by respective bearings 314e and 315e of the first stator 314 and the second stator 315.
The motor in accordance with this proposal is structured such that: there is formed the magnet 311 composed of a cylindrical permanent magnet circumferentially divided into equal parts to be alternately magnetized to different poles; the first coil 312, the magnet 311, and the second coil 313 are sequentially arranged in an axial direction of the magnet 311; the first outer magnetic pole portions 314a and 314b and the first inner magnetic pole portions 314c and 314d, which are excited by the first coil 312, are opposed to the outer circumferential surface and inner circumferential surface of the magnet 311 on one axial side thereof, respectively; the second outer magnetic pole portions 315a and 315b and the second inner magnetic pole portions 315c and 315d, which are excited by the second coil 313, are opposed to the outer circumferential surface and inner circumferential surface of the magnet 311 on the other axial side thereof, respectively; and the output shaft 317 as a rotary shaft is taken out of the cylindrical magnet 311. The motor with this structure can realize higher output and reduction in outer dimensions of the motor. Further, the reduction of the magnet 311 in thickness can reduce the distance between the first outer magnetic pole portion and the first inner magnetic pole portion and the distance between the second outer magnetic pole portion and the second inner magnetic pole portion. Thus, a magnetic resistance in a magnetic circuit can be decreased. Therefore, a large amount of magnetic flux can be generated, thus making it possible to maintain high output, even if a small current is flown through the first coil 312 and the second coil 313.
However, the motor of the type described in Japanese Patent Application Laid-Open No. H09-331666 (U.S. Pat. No. 5,831,356) has a disadvantage that the length in the axial direction is long as in the conventional stepping motor shown in FIG. 17.
As a motor whose axial length is short, there is one shown in, for example, FIG. 20 (refer to Japanese Patent Application Laid-Open Nos. H07-213041 and 2000-50601). This motor is constituted of plural coils 301, 302, and 303 and a disc-shape magnet 304. The coils 301 to 303 each have a thin coin shape, as shown in FIG. 20, and the axes of the coils are arranged parallel to the axis of the magnet 304. Further, the disc-shape magnet 304 is magnetized in an axial direction of the disc, and a magnetized surface of the magnet 304 is arranged to be opposed to the axes of the coils 301 to 303. In this case, as shown by arrows in FIG. 21, the magnetic fluxes generated from the coils 301 to 303 do not completely and effectively act on the magnet 304. Further, as shown in FIG. 21, the center of rotational force acting on the magnet 304 is at the position separated from an outer diameter of the motor by L, and thus, a small torque is generated relative to the motor size. Moreover, a center portion of the motor is occupied by the coils 301 to 303 and the magnet 304, which makes arrangement of other components in the motor difficult. Furthermore, since the plural coils are required, there is a disadvantage in that energization control on the coils is complicated, and that costs rise.
On the other hand, there is known a device for driving diaphragm blades, a shutter, a lens, or the like with the use of the above-described motor of Japanese Patent Application Laid-Open No. H09-331666 (U.S. Pat. No. 5,831,356) or the like. However, the motor of this type has an elongate cylindrical shape. Thus, when used as a driving source for the diaphragm blades, shutter, lens, or the like, the motor needs to be arranged to be in parallel with an optical axis within a lens barrel of a camera. Therefore, a radial dimension of the lens barrel has the total value of not only a radius of a lens and a radius of a throttle opening but also a diameter of the motor.
FIG. 22 is a diagram for explaining the size of a cross section of a lens barrel base plate or light amount adjusting device in the case of using the cylindrical stepping motor as shown in FIG. 19. In FIG. 22, the motor is represented by symbol M; the lens barrel base plate or light amount adjusting device, 400; an opening portion, 401; a diameter of the motor M, D1; a diameter of the opening portion 401, D2; and a diameter of the lens barrel base plate or light amount adjusting device 400, D3. Based on the above, the diameter D3 of the lens barrel base plate or light amount adjusting device 400 is at least more than (2×D1+D2). When the motor shown in FIG. 17 is used, the diameter D1 of the motor M corresponds to the total of the diameters of the coil, magnet, and stator, which means that the diameter D3 of the light amount adjusting device 400 becomes extremely large.
Further, in the case of the motors of the types described in FIGS. 17 and 19, the position where the magnetic flux generated through energization of the first coil acts on the magnet deviates from the position where the magnetic flux generated through energization of the second coil acts on the magnet in the axial direction of the magnet. Therefore, in the case where nonuniformity in magnetization exists between the positions in a direction parallel to the axis (that is, the position on the 314 side and the position on the 315 side in FIG. 19), the accuracy of the rotation stop position of the magnet may deteriorate.
In view of the above, the applicant of the present invention has proposed a motor that solves the above-mentioned problems (refer to Japanese Patent Application Laid-Open No. 2003-23763 (U.S. Pat. No. 6,591,066)). The motor is provided with: a rotatable rotor having a cylindrical magnet, which is divided into equal portions in a circumferential direction to be alternately magnetized to different poles; a first outer magnetic pole portion which is excited by a first coil and opposed to an outer circumferential surface of the magnet within a first predetermined angle range; a first inner magnetic pole portion which is excited by the first coil and opposed to an inner circumferential surface of the magnet; a second outer magnetic pole portion which is excited by a second coil and opposed to the outer circumferential surface of the magnet within a second predetermined angle range; and a second inner magnetic pole portion which is excited by the second coil and opposed to the inner circumferential surface of the magnet. In the motor, the first outer magnetic pole portion and the second outer magnetic portion are arranged on the same circumference with the magnet as the center.
Although the motor disclosed in Japanese Patent Application Laid-Open No. 2003-23763 is not susceptible to an influence of nonuniformity in magnetization of the magnet and has a short axial length, the motor has a structure in which all the outer magnetic pole portions are arranged in the inner circumferences of the coils. Thus, there has been a problem in that, when the outer diameter of the motor is to be reduced, the range in which the outer magnetic pole portion opposes the outer circumference of the magnet is limited (the area where the outer magnetic pole portion does not oppose the outer circumference of the magnet is large), which leads to low output.